1. Field
The invention is in the field of gas purification.
2. State of the Art
Natural gas at the well-head is usually admixed with undesirable impurities such as water, hydrogen sulfide, carbon dioxide, and nitrogen. Prior to introduction of the gas into a pipeline it is normally necessary to decrease the amount of such impurities to prescribed values. Nitrogen is not a problem in most natural gas fields, since it is usually present in only small quantities that are well below the prescribed value. However, in some fields objectionable quantites of nitrogen are present, such as the Westbrook field in Texas, the Pennsylvania reservoirs in Oklahoma, some fields in the Texas Panhandle, and some others.
The usual procedure for purging nitrogen from the natural gas stream consists of an absorption/desorption process wherein the natural gas is flowed countercurrently to an absorbent which preferentially absorbs the nitrogen as compared to the hydrocarbons in the natural gas. Thus, at least a partial separation of the nitrogen and the natural gas is achieved.
One such procedure employs liquid ammonia as an absorbent which preferentially absorbs nitrogen as described in U.S. Pat. No. 2,521,233. However, this procedure is costly and hazardous since it involves the use of liquid ammonia, low temperatures in the range of -30.degree. F. to -100.degree. F., and high pressures in the range of 700 psia.
Another procedure is that described in U.S. Pat. No. 3,977,203, wherein nitrogen along with other impurities is removed by a liquid/liquid extraction process using a polar solvent. This process is also very costly since it involves a great deal of expensive equpiment.
Still another procedure is that described in U.S. Pat. No. 4,832,718, wherein the natural gas is dissolved in a solvent and the solution is later flashed to effect separation. However, the solvent disclosed has a relatively high vapor pressure thus leading to excessive loss or, alternatively, requiring an additional recovery operation.
Other procedures employed in the prior art have relied upon the use of temperatures either significantly higher or lower than ambient, pressures abnormally high, costly equipmeent, and/or secondary purge gas extraction.
It was not previously recognized that by using an essentially inverse process, whereby the desired product, i.e., natural gas, was absorbed in an absorbent preferentially as compared to nitrogen, and recovered by desorption, and wherein the absorbent had a very low vapor pressure and a high boiling point, that sucessful purging of the nitrogen could be achieved utilizing only ambient temperature, modest pressures, and simple inexpensive equipment.
Therefore, it was an object in the making of the invention to provide a process whereby nitrogen could be substantially purged from the natural gas stream utilizing only simple inexpensive equipment, and which would not involve the use of cryogenic temperatures, high temperatures, abnormally high pressures, nor the use of an absorbent subject to excessive loss during the cycle.